Department Of Energy requests $2.4 billion for renewable energy, efficiency in fiscal year 2011. The proposed budget boosts renewable energy sources and storages application and development, such as wind power, solar power, batteries, and fuel cells and so on. There is the widespread growth of distributed energy systems using these renewable energy sources and storages. More and more attention is paid to the high power density, high power efficiency, high power quality and high system reliability of the distributed energy systems.
The main reason that prevents the grid-connected photovoltaic (PV) systems from realizing its full market potential is the power losses due to the module mismatch, orientation mismatch, partial shading, and maximum power point tracking (MPPT) inefficiencies. The traditional cascaded dc-dc converter topology shown in FIG. 1 can interface distributed PV modules and achieve MPPT for each PV module, therefore reduces the power loss. However, the configuration has dc-dc and dc-ac conversion stages, which decrease the overall system efficiency. Moreover, the switching frequency of dc-ac inverter is limited leading to the big size AC filter and large electrolyte capacitors, which restricts the system power quality and power density. Cascaded multilevel inverter topology such as H-bridge inverter, shown in FIG. 2, can interface distributed PV modules, achieve MPPT for each PV module and single stage energy conversion, support a higher equivalent pulse width modulation (PWM) frequency and a larger DC bus voltage. Nevertheless, the H-bridge inverter lacks boost function so that the inverter kVA requirement has to be increased twice with a PV voltage range of 1:2, which restricts the system power density. In addition, both configurations are not able to be immune to shoot-through faults. Therefore, the modular system meets a daunting challenge.